Smoothing output fluctuations of a wind farm is known. If the standard deviation of the output fluctuation of one wind turbine is defined as σ, then the standard deviation σtotal of the output fluctuation of a wind farm with M wind turbines is represented by the following equation (1) provided that the output fluctuations of each wind turbine are not correlated (random).[Equation 1]σtotal=σ√{square root over (M)}  (1)    Patent Citation 1: U.S. Pat. No. 6,724,097, specification    Patent Citation 2: U.S. Pat. No. 6,946,751, specification